Screw vacuum pump comprising additional flow bodies

ABSTRACT

The invention relates to a screw vacuum pump comprising two rotors ( 4, 5 ) which respectively have a hub ( 6, 7 ) and a thread ( 8, 9 ), additionally comprising a housing ( 2 ) wherein the rotors whose threads engage with each other are accommodated in such a way that they form, together with the housing, inlet cross sections ( 15, 16 ) on the inlet side thereof and form outlet cross sections ( 29 ) on the pressure side thereof causing gas to be conveyed from the inlet side to the pressure side during rotation of the rotors ( 4, 5 ). According to the invention, in order to improve inflow and outflow conditions, the rotors ( 4, 5 ) are provided on the inlet side with flow bodies ( 21, 22; 26, 27, 28; 36, 37 ) which are arranged upstream from the inlet cross sections ( 15, 16 ) in such a way that the inflow conditions of the gas to be transported to the inlet cross sections ( 15, 16 ) are improved.

[0001] The present invention relates to a screw vacuum pump having the characteristics of patent claim 1. A pump of this kind is known from the international patent application WO/00/12900.

[0002] In the instance of a screw pump, threads engaging into each other establish sealed off volumes, which during the synchronised rotation of the rotors are conveyed from the inlet to the outlet. Inlet and outlet are commonly so designed that the thread ridges of the rotors—generally single threads—commence, respectively terminate, in a plane perpendicular to the rotor axes. The effective inlet cross-section (respectively outlet cross-section) of the active pumping elements for this reason corresponds in each instance to the total of the surfaces which form the respective hub of the rotors, the housing, respectively—depending on the position of the rotor—the adjacent rotor, as well as the limits at the side of respective the thread's ridge. In single threads, the inlet cross-section extends in each instance over 180°.

[0003] Drawing FIGS. 1 and 2 depict a rotor inlet according to the state-of-the-art in which the rotors are equipped with single threads. In the drawing FIGS. 1 and 2, the only partially depicted screw vacuum pump is designated as 1, its housing as 2, its inlet as 3, the rotors as 4 and 5, the rotor hubs as 6 respectively 7, their thread ridges as 8 respectively 9, and the rotor axes as 10 respectively 11. In drawing FIG. 2, a developed view of the rotor 5 is depicted.

[0004] The two thread ridges 8, 9 commence in a plane extending perpendicularly with respect to the rotor axes 10, 11, said plane being designated as 14 in both drawing figures. Thus there results for each rotor an inlet cross-section 15 respectively 16, being created by the involved components, which extends—in the instance of single thread ridges 8, 9—over 180°.

[0005] It is the task of the present invention to improve the conditions for the inflow and also the outflow in the instance of a screw vacuum pump.

[0006] This task is solved through the present invention by the characterising features of the patent claims.

[0007] Through the present invention an “inlet booster” is implemented. The arrangement of flow bodies upstream of the inlet cross-sections has the effect of improving the filling degree of the volumes conveyed by the rotors from the inlet to the outlet, so that a pump designed in accordance with the present invention will have improved pumping properties, in particular an improved pumping capacity. Also in the area of the rotor outlets likewise designed flow bodies assigned to the outlet cross-sections can improve the conditions of the outflow, such that the flow losses in the exhaust system can be reduced. Aerodynamically, flow bodies arranged on the delivery side are capable of reducing the flow velocities and the residual swirl, and also the static pressure may be increased additionally through a widening cross-section, so that in the downstream exhaust system lower flow losses occur due to deflection and friction. Since in the exhaust area the back pressure is in any case continuously at 1 bar, the aerodynamic improvements may here be also effective across the entire operating range of the screw pump. Due to the aforementioned benefits, there finally also exists the possibility of employing shorter rotors.

[0008] The present invention may be implemented independently of the geometrical arrangement of the screw (single-thread or multi-thread screws, constant or variable pitch, variable pitch with areas of constant pitch, cylindrical, stepped or cone-shaped rotors, single or double flow rotors, cantilevered rotors or rotors with double sided bearings).

[0009] An advantageous further developed embodiment of the present invention is such, that the thread ridge of the adjacent rotor in each instance (second rotor) is equipped in the area which interacts with the flow body or flow bodies of the first rotor, with a recess.

[0010] Thus closing of the inlet cross-section of the first rotor is delayed while simultaneously reliably filling the increased inlet volume due to the recess. In this manner a pre-compression is effected which improves efficiency of the pump and reduces its power requirements²).

[0011] A further advantage of the present invention is such, that the flow bodies may simultaneously be employed as masses for balancing. Imbalances of the rotors which are unavoidable owing to the design of the end areas of the threads, can be completely or at least to a substantial extent removed through the flow bodies. Even in the instance of the rotors being manufactured by casting, only fine balancing will be required. With regard to rotor dynamics, flow bodies on the outlet side offer the possibility of reducing the initial imbalance³) of a screw rotor additionally in a second plane by calculative/design means and to utilise these then also as the second compensation plane during fine balancing, such that the inner moments in the entire rotor may be minimised.

[0012] The outlet contours may also be applied to all screw geometries. Through the reduced cross-sectional areas in the thread of the screw, only a small wall thickness remains in the instance of threads in which the ridge width reduces at the pressure side of the rotor, whereby said small wall thickness does not leave much room for designing blade contours. Of course almost any outlet contour may be added through an additional part, but post-forming an additional thread by metal cutting, being viable in the instance of a vacuum screw with variable pitch owing to the large pitch on the inlet side, may be employed on the outlet side only in rare cases. It might be conceivable, after providing corresponding slots along the diameter of the hub, to produce through well controlled bending of the remaining thin walled contour, the shape of the blade, which by means of a solid material joint (through welding, soldering or gluing) would then again have to be affixed to the hub. It is more advantageous to manufacture this geometry directly during the manufacture of the thread, so as to obtain a cost-effective and operationally reliable contour which, in addition, may be optimally adapted to rotor dynamic requirements.

[0013] The integral manufacture of screw geometry and inlet and outlet contours through metal cutting operations offers a further benefit. Through facing, perpendicularly with respect to the rotor axis, there result in the instance of a conventional screw rotor sharp inlet and outlet edges at both ends, which frequently need to be cut back in order to prevent the remaining thin materials from being deformed or breaking off. In contrast to this in the instance of integrally manufactured contours, a steady transition may be attained, which simultaneously improves stiffening of the edges at the ends.

[0014] Further advantages and details of the present invention shall be explained with reference to the examples of embodiments depicted in drawing FIGS. 3 to 8.

[0015] Depicted are in

[0016] drawing FIGS. 3, 4 and 8, solutions with one flow body in each instance,

[0017] drawing FIGS. 5, 6 and 7, solutions with several flow bodies in each instance.

[0018] In the example of an embodiment in accordance with drawing FIGS. 3 and 4 (drawing FIG. 4 again depicts a developed view of the rotor 5) the rotor hubs 6, 7 have been extended beyond the plane 14 of the inlet cross-sections 15, 16 by an amount equivalent to the width of one or two thread ridges. Said rotor hubs serve the purpose of supporting one each flow body 21, 22 each located above the inlet cross-sections 15 respectively 16, also limiting the pump chamber on the side of the hub. This is approximately an extension of the thread ridges 8, 9 with reduced ridge width (approximately ⅓). In the instance—as depicted—of single threads, each flow body extends over just under less than half the rotor circumference and subsequently there is just over half the rotor circumference available to the open partial area. Turned by 180° with respect to each other, each of the flow bodies engages in each instance in a non-contact manner into the corresponding gap of the adjacent rotor. The slope of the in each instance forerunning edges of the flow bodies 21, 22 increases slightly in the direction of the intake side. The area of the ends is rounded off. The gases flowing into the still open volume which is to be conveyed, are indicated in drawing FIG. 4 by arrows.

[0019] The areas of the face sides of the thread ridges 8, 9 running behind the flow body 21, 22, are equipped with the recesses 23 (rotor 4, not visible), 24. They delay sealing off of the pumped volumes and will ensure simultaneously that these are completely filled.

[0020] The flow body 21 respectively 22 in each instance, may be manufactured with its hub section as a separate part, and may be retrofitted to the cut off face surface of the screw. However, especially advantageous is an integral manufacture in which the hub section and the flow body are formed by milling, for example, and specifically from the remaining material which has been left over (depicted by dashed lines in drawing FIG. 4) in the production of the screw profile (by milling, spinning, rolling, turning etc.).

[0021] Drawing FIG. 5a depicts an embodiment corresponding to that of drawing FIG. 4 with the difference, that the width and pitch of the ridge 9 decrease in the direction of the pressure side. In an embodiment of this kind, the pressure side may be designed in accordance with drawing FIG. 5b. The hub 7 extends beyond the outlet cross section 29 by about four times the width of the thread ridge on the pressure side and supports a blade-like extension 25 of the thread 9. This extends with strongly increasing slope and ridge width in the direction of the pressure side over approximately 140°.

[0022] Drawing FIG. 6a depicts by way of a developed view, the rotor inlet of a further example of an embodiment for the rotor 5. The not depicted rotor 4 is designed accordingly. Ahead of the inlet cross section 16 there are located independent flow bodies 26, 27, 28 being independent of the thread ridge 9. These are supported at the hub 7 and exhibit approximately the shape of rotor blades, their slope increases in the direction of the intake side, specifically commencing approximately with the slope of thread ridge 9.

[0023] The drawing FIGS. 6b and 6 c depict two embodiments for the rotor outlet, depending on whether the thread 9 has a constant pitch and ridge width or a decreasing pitch and ridge width. On the pressure side, the hub 7 is in each instance extended beyond the outlet cross-section 29 and carries the blades 31, 32, 33 respectively 34, 35. Said blades are independent of thread 9 and have a slope which increases in the direction of the pressure side. In the embodiment in accordance with drawing FIG. 6b, the blades are designed to be approximately mirror symmetrical with respect to the blades 26, 27, 28. In the embodiment in accordance with drawing FIG. 6c, the ridge width of the blades 34, 35 increases in the direction of the pressure side. In these embodiments, the blades on the inlet side and the outlet side with their hub sections consist expediently of separately manufactured rings, which after having been fitted to the face side, are components of the rotors 4, 5. This solution allows to adapt the conditions for the inflowing flow—and under certain conditions—also the conditions for the outgoing flow in a simple manner, by exchanging the blade rings in accordance with customers requirements.

[0024] The flow bodies 25 (drawing FIG. 5b) and 34 (drawing FIG. 6c) on the pressure side have a relatively large volume. Thus in the outlet area of the pump a sufficient mass is available for balancing the rotors.

[0025] In the embodiment in accordance with drawing FIG. 7a, two flow bodies 36, 37 are provided. Flow body 36 is—substantially like in the embodiment in accordance with drawing FIGS. 3, 4—an extension of the thread ridge 8 but reduced in width (here approximately ⅕). The base of the blade shaped flow body 37 is located approximately at the centre of the inlet cross section 16. In an embodiment with a thread 9 of constant ridge width and pitch, the rotor outlet may be designed correspondingly (approximately a mirror image).

[0026] Drawing FIG. 7b depicts the rotor outlet in an embodiment with a thread 9, the pitch and ridge width of which decreases in the direction of the pressure side. In the area of the extension of the hub 7 beyond the outlet cross section 29, the pitch of the thread increases strongly whereby the ridge width is further reduced in the direction of the pressure side.

[0027] Finally, drawing FIG. 8 depicts by way of a perspective view an embodiment which substantially corresponds to the embodiment in accordance with drawing FIGS. 3, 4. The difference is, that the hubs 6, 7 only extended in the area of the flow bodies 21, 22. They extend in each instance only up to the inside edges of the respective flow bodies 21, 22.

[0028] It is expedient to design the flow bodies such that, be it with respect to their design, arrangement and/or mass, they simultaneously remove the imbalance of the screw rotors 4, 5. Beneficially, especially at the location where the arrangement of aerodynamically effective flow bodies is expedient, it is required also to add balancing masses. Great initial imbalances are thus avoided, involved balancing processes can be dispensed with. For this reason, the flow bodies may also be considered as balancing weights, being so designed that they improve the conditions for the inflow (respectively outflow) of the gases to be pumped, i.e. that they have the shape of flow bodies. 

1. Screw vacuum pump comprising two rotors (4, 5) which respectively have a hub (6, 7) and a thread (8, 9), additionally comprising a housing (2) wherein the rotors whose threads engage with each other are accommodated in such a way that they form, together with the housing, inlet cross sections (15, 16) on the inlet side thereof and form outlet cross sections (29) on the pressure side thereof causing gas to be conveyed from the inlet side to the pressure side during rotation of the rotors (4, 5), wherein the rotors (4, 5) are provided on the inlet side with flow bodies (21, 22; 26, 27, 28; 36, 37) which are arranged upstream from the inlet cross sections (15, 16) in such a manner that the conditions for the inflow of the gases to be conveyed to the inlet cross sections (15, 16) are improved.
 2. Pump according to claim 1, wherein in the instance of multi-thread screws, at least one thread ridge is equipped with a flow body.
 3. Pump according to claim 1, wherein the flow body extends in the instance of a single thread screw over 90° to 180°.
 4. Pump according to one of the claims 1, 2 or 3, wherein the flow body (21, 22) substantially is an extension of the ridge (8, 9) with reduced ridge width.
 5. Pump according to one of the claims 1 to 4, wherein there is located in the area of the respective inlet cross-sections at least one further flow body independent of thread ridge (8, 9), said flow body having the shape of the blade.
 6. Pump according to claim 5, wherein the blades are curved such that they extend on the pressure side approximately in the direction of the thread ridges (8, 9) and are designed to be steeper on the intake side.
 7. Pump according to one of the above claims, wherein in the area running behind the face side of the thread ridge (8, 9) there is present a recess (23, 24).
 8. Pump according to one of the above claims, wherein the rotor outlet is equipped with corresponding flow bodies.
 9. Pump according to one of the above claims, wherein the flow body/bodies (21, 22; 26, 27, 28; 31, 32, 33; 34, 35; 36, 37) and the corresponding hub section (6, 7) are fitted by way of a separate component to the face side respectively the rear side of the rotor (4, 5).
 10. Pump according to one of the above claims, wherein the flow bodies with respect to their design, arrangement and/or mass are so designed that they substantially remove the imbalance of the related rotor (4, 5)
 11. Screw vacuum pump comprising two rotors (4, 5) which respectively have a hub (6, 7) and a thread (8, 9), additionally comprising a housing (2) wherein the rotors whose threads engage with each other are accommodated in such a way that they form, together with the housing, inlet cross sections (15, 16) on the inlet side thereof and form outlet cross sections (29) on the pressure side thereof causing gas to be conveyed from the inlet side to the pressure side during rotation of the rotors (4, 5), wherein the rotors (4, 5) are provided on the pressure side and/or inlet side with balancing weights (21, 22; 26, 27, 28; 31, 32, 33; 34, 35; 36, 37) which are designed in such a manner that they improve the conditions for the inflow (respectively outflow) of the gases to be conveyed.
 12. Pump according to claim 11 wherein the balancing weights exhibit the shape of flow bodies in accordance with one of the claims 1 to
 10. 